This invention relates generally to the hydraulic fracture stimulations of subterranean formations. In one aspect, it relates to determining closure pressure of hydraulically induced fractures in formations by pulse testing.
Hydraulic fracturing is a production stimulation technique involving the pumping of a hydraulic liquid down a wellbore and into a subterranean formation at such a pressure and rate to cause the formation to crack (fracture). In the vast majority of such treatments, the fracture is vertical, extending outwardly into the formation from the wellbore. During the latter stages of a fracturing treatment, a particulate propping agent (proppant) is generally deposited in the fracture. When the injection pressure is released, the formation walls close on the propping agent creating a xe2x80x9cpropped fracturexe2x80x9d which provides a high conductivity channel in the subterranean formation. The conductivity of the propped fracture is the product of fracture width and fracture permeability. Permeability can be estimated by the size of the proppant. However, in order to generate sufficient fracture width, it is necessary to obtain a tip screenout (TSO) in the formation. Obtaining a TSO at the correct time is heavily dependent upon an accurate estimate of the fluid leakoff coefficient (CL)xe2x80x94the rate at which fluid leaks off from the fracture to the surrounding rock. It is known that an accurate measurement of CL is based on an accurate determination of fracture closure time (tc), which, in turn, is based on fracture closure pressure (Pc).
For a given volume of fluid pumped into a fracture (V1), the fracture will require a specific amount of time to close (tc), depending on how quickly the fluid filling the fracture leaks off to the surrounding formation. The fracture closes only after all the fluid filling the fracture leaks off. As the fluid leaks off, the gradual closure of the fracture is accompanied by a gradual decline of the pressure inside the fracture. The time required for the fracture to completely close (tc) coincides with the fracture closure pressure (Pc). The fluid volume (V1) and tc are used in subsequent computer simulations for designing the main propped fracture stimulation treatment. Only by accurately determining Pc, can tc be determined, which, in turn, is used to calculate CL.
Thus, an accurate determination of Pc is a key to the design of a fracture treatment. The most common techniques for the onsite determination of Pc are pressure decline analysis, constant-rate flowback testing, and pulse testing.
Pressure decline analysis (briefly alluded to above) involves creating a fracture using a known volume of fluid (V1) pumped at a constant rate. After pumping is complete and the pumps are shut down, the pressure in the fracture will decline as the fluid in the fracture leaks off to the surrounding formation. In many instances, plotting the declining pressure versus the square-root (SQRT) of the elapsed time since pump shut-down (dt) results in a curve with 2 linear sections of different slopes. The intersection of the 2 linear sections is the point of fracture closure and, thus, defines the values of Pc and tc (see FIG. 3 for an example). In some cases the pressure vs. SQRT (dt) plot (i.e., the pressure decline plot) does not yield a clear slope change for determining Pc and tc. In these instances, other methods must be used to determine PC, which can then be used with the original pressure decline plot to determine tc. The most common of these alternate methods are constant-rate flowback testing and pulse testing.
Constant-rate flowback testing involves creating a fracture followed by flowing fluid back from the fracture at a constant rate. This method results in a relatively slow drop in pressure while the fracture is open followed by a more rapid drop in pressure once the fracture closes. This test works well if the flowback rate is held constant during the test. Maintaining a constant flowback rate, however, is sometimes very difficult when applying this method.
A recent SPE publication, SPE Production and Facilities (August 1996), by C. A. Wright, et al., describes the use of fluid pulse testing for determining Pc. Wright""s concept of pulse testing involves creating a fracture followed by pumping small fluid pulses intermittently as the fracture gradually closes. The pressure response from each fluid pulse is analyzed to determine if the fracture is open or closed at the time the pulse was pumped. The pressure response of an open fracture is different than that of a closed fracture. This method is robust and can be easily utilized in most situations. This method, however, determines only a range of possible Pc""s and not a specific Pc.
There is a need for a robust, on-site technique that is similar to the pulse testing technique described above, but can actually determine a specific, singular value of Pc rather than only range of possible Pc""s. Being able to determine a specific value of Pc with such a test would increase the accuracy of determining CL (a critical variable in the design of fracture stimulation treatments).
In its broadest aspect, the method of the present invention involves four main steps:
(a) pumping a large volume of fluid (V1), relative to step (c), down a wellbore and into a subterranean formation to form a fracture therein for data;
(b) permitting the fracture to close;
(c) pumping a small volume of fluid (V2), relative to step (a), into the formation to reopen the fracture more narrowly than in step (a); and
(d) shutting down the pumping operation and determining the pressure in the wellbore at shutdown.
As described below, the pressure determined in step (d) is the initial shutin pressure (ISIP) and is a very close approximation of Pc. The Pc is used to determine the tc in the pressure decline analysis technique (if tc cannot be clearly determined from the pressure decline plot itself). The tc is used, in turn, to determine CL. The value of CL is then used to design the fracture treatment, along with other essential data, using known computer simulation techniques.
In a preferred embodiment, the process may include additional steps of (a) using an initial water breakdown to insure open perforations and that adequate injection rates can be obtained, and (b) proppant scouring prior to fracture generation with a low density proppant slug to scour tortuous fracture paths and help plug multiple fractures.
The variables involved in carrying out the four steps may range within wide limits depending on the factors including formation thickness, formation tensile strength, formation toughness, formation pressure, and pumping equipment, etc.
However, the following parameters are important for the success of the present invention: (1) the data fracturing step must generate a fracture of larger dimensions than the small volume pulse testing and (2) the pulse testing step should be at low volume (e.g. 0.5 to 3 bbls per pulse) and low injection rates (e.g. 2 to 5 bpm).